To whom did the LORD say, "Do you seek great things for yourself? Stop seeking!"?

Answer: Baruch

Through Jeremiah, God told Baruch son of Neriah not to seek great things, but promised to spare his life wherever he went.

“This is the word that Jeremiah the prophet spoke to Baruch son of Neriah when he wrote these words on a scroll at the dictation of Jeremiah in the fourth year of Jehoiakim son of Josiah, king of Judah:”— Jeremiah 45:1 (BSB)